Although both grating and interference filters have been used as optical filters for wavelength division multiplexing, neither provides for sufficiently high wavelength selectivity to effectively make use of the potential optical communication capacity inherent in the available optical bandwidth of fibers. For example, it is possible with relative ease to modulate present semiconductor laser diodes at frequencies up to 1 GHz. Since higher modulation rates entail excess cost penalties the modulation rate of 1 GHz may be adopted as typical of what will be used for a broad range of communication purposes in the near future. With this channel bandwidth, separating adjacent optical channels by a frequency difference much greater than this value is tantamount to wasting optical communication capacity. Yet 1 GHz at one micron wavelength represents a wavelength difference of one 0.003 nm.
Consider a diffraction grating having 1/D=5000 lines/cm. At one micron the first order diffraction angle is given by EQU D n sin .THETA.=.lambda..THETA.=sin.sup.-1 (1/3)=0.339 radians.
Assume further that a compact bulk optic device has dimensions of the order of 1 cm. Then the resolution in angle of the grating is given by, with w, the width of light beam,=1 cm. ##EQU1##
Therefore, the angular resolution of the grating is one part in 5000 or 0.2 nm, i.e., about 100 times larger than is desired for close packed wavelength division multiplexing. Clearly, a grating of sufficient resolution must be 33 cm in size and hence bulky and prohibitively costly.
The best interference filters have resolutions of approximately 1 nm or about 300 times coarser than desired.
There are a very limited number of optical structures which provide the necessary selectivity. The Michelson echalon grating, the Lummer Gehrke plate and the Fabry-Perot etalon are well-known examples. Of these, the Fabry-Perot is unique in that its effective physical length is multiplied by the "finesse" of the etalon. That is the length over which interference is active is equal to the number of round trip distances the light beam bounces back and forth within the etalon before leaking away or being absorbed. The Fabry-Perot etalon is therefore a compact device having extraordinarily high resolution.
Fabry-Perot resonators exhibit many resonances separated in frequency by the amount f where, ##EQU2## denoted as the "free spectral range" , where L is the round trip distance in the resonator, n is the index of refraction and c is the velocity of light in vacuum. The half height, full bandpass of the resonator is defined to be equal to the free spectral range divided by the finesse. For example, a 1 cm thick etalon made of glass having an index of 1.5 has a free spectral range of 10 GHz. If the resonator finesse is made to be equal to 100 then the filter bandpass is equal to 100 MHz. The finesse of the etalon is controlled or determined by the reflectivity of the surface mirrors, the absorption of the internal etalon medium, diffraction losses, and lack of perfect parallelism of the opposing mirror surfaces. With care, parallel plate glass etalons may be manufactured having finesses of up to at least 100.
By virtue of its high finesse, the Fabry-Perot, unlike the Michelson or Mach Zehnder interferometers, allows one to distinguish between a number of different wavelengths bands equal to the value of the finesse of the etalon. For example, if the Fabry-Perot finesse is 100, then in principle one can distinguish between any one of 100 adjacent wavelength bands. However, for use as a filter, one must separate adjacent channels by 3-5 times the bandpass to achieve acceptable crosstalk levels.
However, wavelengths separated by an integer number of the free spectral ranges of the etalon can not be distinguished or separated from one another by a (single) Fabry-Perot etalon. The presence of multiple resonances in effect limits the communication capacity of a single Fabry-Perot etalon to a single free spectral range because of this inability to discriminate modular the free spectral range. While at first sight this appears to be a disadvantage to the approach of using a Fabry-Perot etalon or the similar behaving ring resonators as filters, the ambiguity may be resolved by using, for example, more than one Fabry-Perot resonator in tandem, creating the effect of a much increased free spectral range. With multiple resonators working in vernier fashion the free spectral range is multiplied by the finesse of each additional resonator used for filtering. Thus, for example, if two resonators are used each having a finesse of 100 and a free spectral range of 10 and 10.1 GHz respectively, then the total effective free spectral range is increased from 10 GHz to 1000 GHz. In this case the overlap of resonances from each filter occurs only after 99 or 100 free spectral ranges of the two component filters.
Conversely, the multiple resonances of Fabry-Perot resonators have the benefit not only of vernier tuning but (1) allowing the use of laser operating with frequency differences separated by many free spectral ranges (that is, the need to match laser frequencies is greatly alleviated for single resonator filter systems), and (2) the presence of multiple resonances allows one to transfer the frequency stability of a highly stable source to the etalon and thence electronically stabilize a laser to any coexisting etalon resonance.
Since a simple Fabry-Perot etalon having a finesse of 100 can selectively pass one wavelength band to the exclusion of the remaining 99 wavelength bands, such an etalon can be used as a multiplexer/demultiplexer to efficiently separate or combine many wavelengths of light. One approach for multiplexing is to successively pass a light beam by 100 Fabry-Perot etalons using each etalon to separate a distinct one of the 100 distinguishable wavelengths from the rest. Such a procedure is made difficult by the requirement that the light strike each etalon at substantially normal incidence. Clearly the manufacture and use of 100 separate etalons for multiplexing and demultiplexing is cumbersome and costly both in terms of manufacturing etalons and the necessary optics and the associated electronics required to stabilize the wavelength of the etalon filters in the presence of changing ambient conditions such as temperature.
What is needed is a relatively compact, rugged, and easily manufacturable device that provides a resolution of the order of 1 GHz and a free spectral range of 100 GHz, allowing approximately 100 channels to be multiplexed and demultiplexed. Specifically, what is implied for demultiplexing is that all wavelengths enter via a common single mode fiber and different wavelengths exit in a spatially separated format so that the separated wavelength components can be separately detected, sent to separate fibers or otherwise separately processed. Moreover, what is needed is a controlled method of separation such that spatial separation is linearly proportional to wavelength separation. However, unlike the diffraction grating a much higher resolution is required for close packed wavelength division multiplexing. It should be appreciated that, due to channel crosstalk considerations, the number of useful channels is approximately equal to the finesse divided by 3 to 5.
It is therefore a primary object of the present invention to provide a wavelength division multiplexing device that satisfies these several requirements.